Networking Working Group L. Ginsberg
Internet-Draft P. Psenak
Intended status: Standards Track Cisco Systems
Expires: April 6, 2020 S. Previdi
Huawei
W. Henderickx
Nokia
J. Drake
Juniper Networks
October 4, 2019
IS-IS TE Attributes per applicationdraft-ietf-isis-te-app-07
Abstract
Existing traffic engineering related link attribute advertisements
have been defined and are used in RSVP-TE deployments. Since the
original RSVP-TE use case was defined, additional applications (e.g.,
SRTE, LFA) have been defined which also make use of the link
attribute advertisements. In cases where multiple applications wish
to make use of these link attributes the current advertisements do
not support application specific values for a given attribute nor do
they support indication of which applications are using the
advertised value for a given link.
This draft introduces new link attribute advertisements which address
both of these shortcomings. It also discusses backwards
compatibility issues and how to minimize duplicate advertisements in
the presence of routers which do not support the extensions defined
in this document.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Draft draft-ietf-isis-te-app October 20199. Security Considerations . . . . . . . . . . . . . . . . . . . 1510. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 1511. References . . . . . . . . . . . . . . . . . . . . . . . . . 1511.1. Normative References . . . . . . . . . . . . . . . . . . 1511.2. Informative References . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 161. Introduction
Advertisement of link attributes by the Intermediate-System-to-
Intermediate-System (IS-IS) protocol in support of traffic
engineering (TE) was introduced by [RFC5305] and extended by
[RFC5307], [RFC6119], and [RFC8570]. Use of these extensions has
been associated with deployments supporting Traffic Engineering over
Multiprotocol Label Switching (MPLS) in the presence of Resource
Reservation Protocol (RSVP) - more succinctly referred to as RSVP-TE.
In recent years new applications have been introduced which have use
cases for many of the link attributes historically used by RSVP-TE.
Such applications include Segment Routing Traffic Engineering (SRTE)
and Loop Free Alternates (LFA). This has introduced ambiguity in
that if a deployment includes a mix of RSVP-TE support and SRTE
support (for example) it is not possible to unambiguously indicate
which advertisements are to be used by RSVP-TE and which
advertisements are to be used by SRTE. If the topologies are fully
congruent this may not be an issue, but any incongruence leads to
ambiguity.
An additional issue arises in cases where both applications are
supported on a link but the link attribute values associated with
each application differ. Current advertisements do not support
advertising application specific values for the same attribute on a
specific link.
This document defines extensions which address these issues. Also,
as evolution of use cases for link attributes can be expected to
continue in the years to come, this document defines a solution which
is easily extensible to the introduction of new applications and new
use cases.
2. Requirements Discussion
As stated previously, evolution of use cases for link attributes can
be expected to continue - so any discussion of existing use cases is
limited to requirements which are known at the time of this writing.
However, in order to determine the functionality required beyond what
already exists in IS-IS, it is only necessary to discuss use cases
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which justify the key points identified in the introduction - which
are:
1. Support for indicating which applications are using the link
attribute advertisements on a link
2. Support for advertising application specific values for the same
attribute on a link
[RFC7855] discusses use cases/requirements for SR. Included among
these use cases is SRTE which is defined in
[I-D.ietf-spring-segment-routing-policy]. If both RSVP-TE and SRTE
are deployed in a network, link attribute advertisements can be used
by one or both of these applications. As there is no requirement for
the link attributes advertised on a given link used by SRTE to be
identical to the link attributes advertised on that same link used by
RSVP-TE, there is a clear requirement to indicate independently which
link attribute advertisements are to be used by each application.
As the number of applications which may wish to utilize link
attributes may grow in the future, an additional requirement is that
the extensions defined allow the association of additional
applications to link attributes without altering the format of the
advertisements or introducing new backwards compatibility issues.
Finally, there may still be many cases where a single attribute value
can be shared among multiple applications, so the solution must
minimize advertising duplicate link/attribute pairs whenever
possible.
3. Legacy Advertisements
There are existing advertisements used in support of RSVP-TE. These
advertisements include sub-TLVs for TLVs 22, 23, 25, 141, 222, and
223 and TLVs for SRLG advertisement.
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Standard Application Identifier Bits are defined/sent starting with
Bit 0. Additional bit definitions that may be defined in the future
SHOULD be assigned in ascending bit order so as to minimize the
number of octets that will need to be transmitted. Undefined bits
MUST be transmitted as 0 and MUST be ignored on receipt. Bits that
are NOT transmitted MUST be treated as if they are set to 0 on
receipt.
User Defined Application Identifier Bits have no relationship to
Standard Application Identifier Bits and are NOT managed by IANA or
any other standards body. It is recommended that bits are used
starting with Bit 0 so as to minimize the number of octets required
to advertise all UDAs.
4.2. Application Specific Link Attributes sub-TLV
A new sub-TLV for TLVs 22, 23, 25, 141, 222, and 223 is defined which
supports specification of the applications and application specific
attribute values.
Type: 16 (temporarily assigned by IANA)
Length: Variable (1 octet)
Value:
Application Identifier Bit Mask
(as defined in Section 4.1)
Link Attribute sub-sub-TLVs - format matches the
existing formats defined in [RFC5305] and [RFC8570]
When the L-flag is set in the Application Identifier Bit Mask, all of
the applications specified in the bit mask MUST use the link
attribute sub-TLV advertisements listed in Section 3.1 for the
corresponding link. Link attribute sub-sub-TLVs for the
corresponding link attributes MUST NOT be advertised for the set of
applications specified in the Standard/User Application Identifier
Bit Masks and all such advertisements MUST be ignored on receipt.
Multiple Application Specific Link Attribute sub-TLVs for the same
link MAY be advertised. When multiple sub-TLVs for the same link are
advertised, they SHOULD advertise non-conflicting application/
attribute pairs. A conflict exists when the same application is
associated with two different values of the same link attribute for a
given link. In cases where conflicting values for the same
application/attribute/link are advertised all the conflicting values
MUST be ignored.
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For a given application, the setting of the L-flag MUST be the same
in all sub-TLVs for a given link. In cases where this constraint is
violated, the L-flag MUST be considered set for this application.
A new registry of sub-sub-TLVs is to be created by IANA which defines
the link attribute sub-sub-TLV code points. This document defines a
sub-sub-TLV for each of the existing sub-TLVs listed in Section 3.1
except as noted below. The format of the sub-sub-TLVs matches the
format of the corresponding legacy sub-TLV and IANA is requested to
assign the legacy sub-TLV identifer to the corresponding sub-sub-TLV.
4.2.1. Special Considerations for Maximum Link Bandwidth
Maximum link bandwidth is an application independent attribute of the
link. When advertised using the Application Specific Link Attributes
sub-TLV, multiple values for the same link MUST NOT be advertised.
This can be accomplished most efficiently by having a single
advertisement for a given link where the Application Identifier Bit
Mask identifies all the applications which are making use of the
value for that link.
It is also possible to advertise the same value for a given link
multiple times with disjoint sets of applications specified in the
Application Identifier Bit Mask. This is less efficient but still
valid.
If different values for Maximum Link Bandwidth for a given link are
advertised, all values MUST be ignored.
4.2.2. Special Considerations for Unreserved Bandwidth
Unreserved bandwidth is an attribute specific to RSVP. When
advertised using the Application Specific Link Attributes sub-TLV,
bits other than the RSVP-TE(R-bit) MUST NOT be set in the Application
Identifier Bit Mask. If an advertisement of Unreserved Bandwidth is
received with bits other than the RSVP-TE bit set, the advertisement
MUST be ignored.
4.3. Application Specific SRLG TLV
A new TLV is defined to advertise application specific SRLGs for a
given link. Although similar in functionality to TLV 138 (defined by
[RFC5307]) and TLV 139 (defined by [RFC6119], a single TLV provides
support for IPv4, IPv6, and unnumbered identifiers for a link.
Unlike TLVs 138/139, it utilizes sub-TLVs to encode the link
identifiers in order to provide the flexible formatting required to
support multiple link identifier types.
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Type: 238 (Temporarily assigned by IANA)
Length: Number of octets in the value field (1 octet)
Value:
Neighbor System-ID + pseudo-node ID (7 octets)
Application Identifier Bit Mask
(as defined in Section 4.1)
Length of sub-TLVs (1 octet)
Link Identifier sub-TLVs (variable)
0 or more SRLG Values (Each value is 4 octets)
The following Link Identifier sub-TLVs are defined. The type
values are suggested and will be assigned by IANA - but as
the formats are identical to existing sub-TLVs defined for
TLVs 22, 23, 25, 141, 222, and 223 the use of the suggested
sub-TLV types is strongly encouraged.
Type Description
4 Link Local/Remote Identifiers (see [RFC5307])
6 IPv4 interface address (see [RFC5305])
8 IPv4 neighbor address (see [RFC5305])
12 IPv6 Interface Address (see [RFC6119])
13 IPv6 Neighbor Address (see [RFC6119])
At least one set of link identifiers (IPv4, IPv6, or unnumbered) MUST
be present. TLVs which do not meet this requirement MUST be ignored.
Multiple TLVs for the same link MAY be advertised.
When the L-flag is set in the Application Identifier Bit Mask, SRLG
values MUST NOT be included in the TLV. Any SRLG values which are
advertised MUST be ignored. Based on the link identifiers advertised
the corresponding legacy TLV (see Section 3.2) can be identified and
the SRLG values advertised in the legacy TLV MUST be used by the set
of applications specified in the Application Identifier Bit Mask.
For a given application, the setting of the L-flag MUST be the same
in all TLVs for a given link. In cases where this constraint is
violated, the L-flag MUST be considered set for this application.
5. Deployment Considerations
If link attributes are advertised associated with zero length
Application Identifier Bit Masks for both standard applications and
user defined applications, then that set of link attributes MAY be
used by any application. If support for a new application is
introduced on any node in a network in the presence of such
advertisements, these advertisements MAY be used by the new
application. If this is not what is intended, then existing
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advertisements MUST be readvertised with an explicit set of
applications specified before a new application is introduced.
6. Attribute Advertisements and Enablement
This document defines extensions to support the advertisement of
application specific link attributes.
Whether the presence of link attribute advertisements for a given
application indicates that the application is enabled on that link
depends upon the application. Similarly, whether the absence of link
attribute advertisements indicates that the application is not
enabled depends upon the application.
In the case of RSVP-TE, the advertisement of application specific
link attributes implies that RSVP is enabled on that link.
In the case of SRTE, advertisement of application specific link
attributes does NOT indicate enablement of SRTE. The advertisements
are only used to support constraints which may be applied when
specifying an explicit path. SRTE is implicitly enabled on all links
which are part of the Segment Routing enabled topology independent of
the existence of link attribute advertisements
In the case of LFA, advertisement of application specific link
attributes does NOT indicate enablement of LFA on that link.
Enablement is controlled by local configuration.
If, in the future, additional standard applications are defined to
use this mechanism, the specification defining this use MUST define
the relationship between application specific link attribute
advertisements and enablement for that application.
This document allows the advertisement of application specific link
attributes with no application identifiers i.e., both the Standard
Application Identifier Bit Mask and the User Defined Application
Identifier Bit Mask are not present (See Section 4.1). This supports
the use of the link attribute by any application. In the presence of
an application where the advertisement of link attribute
advertisements is used to infer the enablement of an application on
that link (e.g., RSVP-TE), the absence of the application identifier
leaves ambiguous whether that application is enabled on such a link.
This needs to be considered when making use of the "any application"
encoding.
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Internet-Draft draft-ietf-isis-te-app October 20197. Interoperability, Backwards Compatibility and Migration Concerns
Existing deployments of RSVP-TE utilize the legacy advertisements
listed in Section 3. Routers which do not support the extensions
defined in this document will only process legacy advertisements and
are likely to infer that RSVP-TE is enabled on the links for which
legacy advertisements exist. It is expected that deployments using
the legacy advertisements will persist for a significant period of
time - therefore deployments using the extensions defined in this
document must be able to co-exist with use of the legacy
advertisements by routers which do not support the extensions defined
in this document. The following sub-sections discuss
interoperability and backwards compatibility concerns for a number of
deployment scenarios.
Note that in all cases the defined strategy can be employed on a per
link basis.
7.1. RSVP-TE only deployments
In deployments where RSVP-TE is the only application utilizing link
attribute advertisements, use of the the legacy advertisements can
continue without change.
7.2. Multiple Applications: Common Attributes with RSVP-TE
In cases where multiple applications are utilizing a given link, one
of the applications is RSVP-TE, and all link attributes for a given
link are common to the set of applications utilizing that link,
interoperability is achieved by using legacy advertisements and
sending application specific advertisements with L-bit set and no
link attribute values. This avoids duplication of link attribute
advertisements.
7.3. Multiple Applications: All Attributes Not Shared w RSVP-TE
In cases where one or more applications other than RSVP-TE are
utilizing a given link and one or more link attribute values are NOT
shared with RSVP-TE, it is necessary to use application specific
advertisements as defined in this document. Attributes for
applications other than RSVP-TE MUST be advertised using application
specific advertisements which have the L-bit clear. In cases where
some link attributes are shared with RSVP-TE, this requires duplicate
advertisements for those attributes.
The discussion in this section applies to cases where RSVP-TE is NOT
using any advertised attributes on a link and to cases where RSVP-TE
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is using some link attribute advertisements on the link but some link
attributes cannot be shared with RSVP-TE.
7.4. Use of Application Specific Advertisements for RSVP-TE
The extensions defined in this document support RSVP-TE as one of the
supported applications. This allows that RSVP-TE could eventually
utilize the application specific advertisements. This can be done in
the following step-wise manner:
1)Upgrade all routers to support extensions in this document
2)Readvertise all legacy link attributes using application specific
advertisements with L-bit clear and R-bit set.
3)Remove legacy advertisements
Migrating RSVP-TE away from legacy advertisements could result in
some implementation simplification as it allows the removal of code
which encodes/decodes the legacy advertisements. Whether this is
seen as desirable is something for the marketplace to determine.
8. IANA Considerations
This document defines a new sub-TLV for TLVs 22, 23, 25, 141, 222,
and 223.
Type Description 22 23 25 141 222 223
---- --------------------- ---- ---- ---- ---- ---- ----
16 Application Specific y y y(s) y y y
Link Attributes
This document defines one new TLV:
Type Description IIH LSP SNP Purge
---- --------------------- --- --- --- -----
238 Application Specific n y n n
SRLG
This document requests a new IANA registry be created to control the
assignment of sub-sub-TLV codepoints for the Application Specific
Link Attributes sub-TLV. The suggested name of the new registry is
"sub-sub-TLV code points for application specific link attributes".
The registration procedure is "Expert Review" as defined in
[RFC8126]. The following assignments are made by this document:
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